1. Field of the Invention
The present invention relates to a photoelectric cell, particularly one that excludes the background, using two either separate light-receiving components or two such components combined in a single device. The light-receiving components provide currents that vary differently depending on the distance of an object or target located in the path of the optical beam received by the components. The invention also uses an electronic circuit that carries out differential processing of the currents.
2. Discussion of the Background
Such cells, using for example a light-receiving device called a Position Sensitive Device (PSD), are well known in the art. They make use of the fact that, for a distance of the object greater than a nominal distance known as the detection range, the first current (provided by the first component) is greater than the second current (provided by the second component) whereas the opposite is the case at distances less than this range. Use is made of this phenomenon by means of an electronic circuit that transforms the currents into signals with different voltages and compares the voltage signals in a main comparator. This comparator provides an output signal that signifies the ON or OFF status of the cell; the cell then interprets the position of the object as being beyond or within the range.
It is found, however, that if the object is at infinite distance or very large the voltages representing the two currents are practically nil. This results in the comparator output being unstable under circumstances where interference is present.